Modern integrated circuits are made up of literally millions of active and/or passive devices such as transistors and capacitors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding. In a typical bumping process, interconnect structures are formed on metallization layers, followed by the formation of under-bump metallurgy (UBM), and the mounting of solder balls.
Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad. An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in that order, on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals. Recently, copper interconnect post technology has been proposed. Instead of using a solder bump, the electrical component is connected to a substrate by means of a copper post. The copper interconnect post technology achieves finer pitch with minimum probability of bump bridging, reduces the capacitance load for the circuits, and allows the electrical component to perform at higher frequencies. A solder alloy is still necessary for capping the bump structure and joining electrical components as well.
Usually, in wet etching the UBM layer, an isotropic etch profile is produced, in which the etching is at the same rate in all directions, leading to undercutting of the etched UBM material. This action results in an undesirable loss of line width. The undercut caused by wet etching process will induce the stress concentration, resulting in bump sidewall delamination, bump crack and bump bridge in fine pitch designs. Although the undercut is an inherent result of the etching process, the undercut is detrimental to the long-term reliability of the interconnection. The undercut compromises the integrity of the solder bump structure by weakening the bond between the solder bump and the bonding pad of the chip, thereby leading to premature failure of the chip.